This invention is related to the class of systems and devices to produce power from solar energy. Related areas include gas engines and compressors.
While solar energy is widely available, current means to harness it are sufficiently expensive that for most applications it is not economically viable. The basic problem is that solar energy is not sufficiently concentrated, and the cost of the collector becomes the limiting factor except in limited applications such as heating household water, swimming pools or intake air for large buildings. The cost of land by itself usually makes solar power non-competitive. However, solar energy is “free”, so if one can build a solar collector cheap enough and installable on land that is also used for another purpose, relatively inefficient systems can be competitive with natural gas based power plants. In practice, this means solar collectors must be flat, act as either the roof or walls of a building or other structure, not leak or contain high pressures or dangerous fluids, and be aesthetic. Photovoltaics can meet these requirement, but material costs by themselves put a floor on the cost of photovoltaic systems that makes them non-competitive with commercial power generation. For a rough comparison, commercially available systems have 7 to 11% efficiency, and cost $4 to $9 per watt of output. As structural material this translates to $40-$100 per square foot. Current roofing material costs range around $1-3/sq foot. Effective power cost ranges between $0.25 and $1/Kwh. In both cases this is at least one order of magnitude too high, maybe even two.
Thermal collectors can be made flat and cheap. There are numerous systems on the market today that can achieve 20-80 degrees Kelvin temperature rise under full sun conditions, and are used to generate hot water or air for space heating. Web page reference [1] (see below) provides a good overview of solar collectors. Reference [2] provides detail on one low temperature application of solar collectors—indoor air heating. Reference [5] is a good review of the state of the art in heat control window glazings. The challenge in harnessing solar power using thermal collectors for non-thermal uses is to get the temperature up to the point where useful amounts of available energy can be produced. For flat plate collectors this is a particular challenge since there is a large area exposed to the environment. Current flat plate collectors tend to lose a lot of the heat collected, because the surface in contact with the environment is heated significantly. Thermal sweep systems (see related patents below) can reduce heat losses to near zero, but current systems are opaque and thus not applicable to a solar collector. So one would like to keep the temperature increment small.
On the other hand, with small temperature increments thermodynamic efficiencies are low. Even worse, since the circulating power is high in such scenarios, small inefficiencies in the components of the conversion equipment translate into relatively large reductions of output power—sometimes even making it disappear altogether. For example, in a Brayton cycle operating at 10% thermodynamic efficiency, the power fed back from the output turbine to the compressor can exceed 9 times the output power. If the turbine+compressor loses 1% (i.e. it is 99% efficient) the output power is reduced 10%. If it loses 10% (a more realistic number) we get no output power at all! Thus we need the components of the heat engine to be super efficient, and we would like to have the temperature increment be as high as possible.
Another issue with solar power is what happens if there is a power demand during the night, or when it is overcast. Photovoltaics can continue to generate power (albeit at a lower level) on overcast days—any alternative system must also be able to do so. Sufficient storage combined with excess capacity can resolve this. Current battery systems by themselves cost enough to blow away the economics of a solar system. As a result, current photovoltaic systems rely on a complicated interface to the electrical grid and a legal requirement on the electrical utility to support “net metering” to effectively store in the utility system the excess power on a sunny day. Because current heat engines need most of the heat input at high end of the temperature range, current thermal storage systems for a days worth of storage are either too large to make sense or use special (expensive) materials to store a lot of heat at the high end of the temperature range using some kind of phase change.